1. Field of the Invention
The present invention provides a method of forming a microcrystalline silicon film at a high rate by high-frequency plasma CVD using as a source gas at least a silicon compound such as silane, a photovoltaic element, and a method of producing the photovoltaic element. The microcrystalline silicon film according to the present invention is suitably applicable to thin film semiconductor elements such as thin film solar cells or thin film transistors.
2. Related Background Art
In the late 1970s valency control of the amorphous silicon based thin film became possible and research and development has been carried out since then to apply the amorphous silicon based thin film to photovoltaic elements typified by solar cells. To produce the photovoltaic element using an amorphous silicon based film or the like, the plasma CVD method using a high frequency (RF) typified by 13.56 MHz has been widely used. The photovoltaic element comprised of the amorphous silicon based film or the like produced by the high-frequency plasma CVD method achieved relatively good photoelectric conversion efficiency with a smaller amount of material, as compared with those of bulk monocrystalline or polycrystalline silicon, but has a problem in terms of the process speed. Specifically, the thickness of the amorphous silicon layer used for an active layer is several thousand .ANG. and the forming rate thereof has to be controlled to a low rate of not more than several .ANG./s in order to obtain a good quality amorphous silicon layer. This made the reduction of process cost difficult.
In the case of plasma CVD using 13.56 MHz that the quality of formed films tend to be degraded abruptly with an increase of the thin film forming rate. The method also had a disadvantage in that it was not easy to increase the throughput in mass production. The plasma CVD method using the microwave (MW) typified by 2.45 GHz is also known as a method capable of forming a thin film with relatively good quality even at relatively large thin film forming rates. An example of formation of an intrinsic layer (i-type layer) by the microwave plasma CVD method includes, for example, "A-Si solar cells by microwave plasma CVD process," Kazufumi Azuma, Takeshi Watanabe, and Toshikazu Shimada, Extended Abstracts (The 50th Meeting); The Japan Society of Applied Physics, p 566 and the like. It is also known that use of the frequency in the VHF band around the frequency of 100 MHz is also effective to improve the quality and increase the speed for formation of amorphous silicon films. For example, U.S. Pat. No. 4,933,203 describes that in the frequency range of 25 to 150 MHz, amorphous silicon films of good quality can be obtained where a ratio f/d of frequency f (MHz) to inter-electrode distance d (cm) is in the range of 30 to 100 (MHz/cm). In the U.S. patent the relation of frequency to inter-electrode distance is specified as to the method of producing the amorphous silicon film, but the patent describes nothing about the method of producing the microcrystalline silicon film nor about specifying the forming pressure in the frequency range of not less than 150 MHz.
Incidentally, the thin film photovoltaic element using the amorphous silicon based thin film generally has a p-i-n junction structure and photoelectric conversion occurs principally in the i-type layer. Many attempts have been made for microcrystallizing the p-type layer and n-type layer in order to improve the junction characteristics between the p or n and i layers. For example, Japanese Patent Application Laid-Open No. 57-187971 discloses a method of increasing output current and output voltage by forming the i-type layer of amorphous silicon and forming at least the layer on the light incident side out of the p-type layer and n-type layer of microcrystalline silicon having an average grain size of not more than 100 .ANG.. It is, however, difficult at present to suppress the phenomenon in which the defect density of the i-type layer increases during exposure to light to lower the photoelectric conversion efficiency (the so-called Staebler-Wronski effect), regardless of the forming method, in the case of the pin type solar cells using amorphous silicon for the i-type layer. This has been a major problem in practical use.
In recent years, attempts have been made to use i-type microcrystalline silicon for the photoelectric conversion layer of the amorphous silicon based thin film photovoltaic element. For example, the group of Shah et al. in Neuchatel University reported a pin type microcrystalline silicon solar cell without optical damage and with the photoelectric conversion efficiency 7.7% in which all the layers of p-layer, i-layer, and n-layer were made of microcrystalline silicon, at 25th IEEE PV Specialists Conference, Washington, May 13-17, 1996. The method of forming the microcrystalline silicon i-layer, employed by the group, was basically the same as the configuration of the conventional high-frequency plasma CVD method and did not use a high-temperature process of not less than 500.degree. C. necessary for formation of the crystalline silicon thin films, such as the polycrystalline silicon thin film. In addition, the VHF-band frequency of 110 MHz is adopted as the plasma forming frequency.
As described above, the pin type solar cells using the i-type microcrystalline silicon film formed by the VHF-band frequency have an advantage of not being optically damaged while being capable of being produced in the low-temperature process. According to the aforementioned report by the group of Shah et al. of Neuchatel University, the deposition rate of the microcrystalline silicon i-type layer was 1.2 .ANG./s and the thickness thereof was 3.6 .mu.m. It is seen from simple computation, that the time to form the microcrystalline silicon i-type layer is as long as eight or more hours. Although the conversion efficiency is relatively high and optical damage does not occur, the throughput is very small; as a consequence, it becomes difficult to reduce the cost. It is essential to drastically increase the forming rate of the microcrystalline silicon i-type layer in order to make the mass production of pin solar cells using microcrystalline silicon for the i-layer practical. It was, however, relatively recently that solar cells with relatively good photoelectric conversion efficiency were produced in the form of the pin type solar cells using microcrystalline silicon for the i-layer. Presently, little is known about the technology for increasing the forming rate of the i-type microcrystalline silicon layer. It is expected that, for example, use of a high-temperature process of not lower than 500.degree. C. permits energy for crystallization to be obtained as thermal energy from the substrate and the increase of the film forming rate can be achieved thereby relatively easily. However, the use of the high-temperature process will pose problems of degradation of element characteristics due to mutual diffusion at the interfaces between the layers such as p/i and n/i, etc., as well as increase of the process cost, and so on.
The present invention concerns the above problems. Specifically, an object of the present invention is to provide a method of forming a microcrystalline silicon film suitable for the i-type layer of the pin type solar cell, at a high rate without using the high-temperature process. Another object of the present invention is to provide a method of forming a microcrystalline silicon film at the high rate of 2 to several ten .ANG./s, notwithstanding use of a low-temperature process of 150 to 500.degree. C.